Hydrogen-rich Inclusion Compounds at High-pressure

Molecular hydrogen (H₂) has been proposed as an alternative fuel source for vehicles. Though H₂ has many benefits, such as clean combustion and the highest known energy density by mass, there are issues in how to store it in a safe and cost effective way. One solution is to store hydrogen in a chemical compound, and gas clathrates (crystalline inclusion compounds) have shown promising results. Pressure provides a powerful means to tune the properties of such compounds and its effects on potential hydrogen storage materials are widely explored. We have recently developed a hydrogen-compatible gas loader for the Paris-Edinburgh press, which enables the loading of high density hydrogen into a clamp with a sample volume suitable for neutron diffraction experiments using the Paris-Edinburgh press [1]. Neutron diffraction is the technique of choice for such materials since it can reveal the location and occupancy of the hydrogen sites. We will present recent data from high-pressure neutron diffraction experiments on hydrogen hydrates as well as other clathrate forming systems like urea and hydroquinone.

Gas hydrates at high pressure

The behaviour of gas hydrates at high pressure is of wide interest and importance. Gas hydrates are stablised by water-gas repulsive interactions. Information on the effect of changing density on these water-gas interactions provides fundamental insight into the nature of the water potential. Gas hydrates are also widely found in nature and systems like the ammonia-water and methane-water systems form the basis of 'mineralogy' of planetary bodies like Saturn's moon Titan. Finally, gas hydrates offer the possibility of cheap environmentally inert transportation and storage for gases like carbon dioxide and hydrogen. We have been carrying out investigations of a range of gas hydrates at high pressure using neutron and x-ray diffraction as well as other techniques. Results from these studies including; the phase diagram of the ammonia water system, the occupancies of hexgonal clathrate structures, and new structures in the carbon dioxide water system, will be presented.